Prepulling mobs as healer

[Vnolan]

Why is prepulling considered rude/bad in this case?

Power-tripping tanks.

Yet several times I've heard tank or dps get mad and tell me to stop prepulling, even when they had ignored my request to pull more.

DPS have no say in the matter and as you mention, the tank is already using AoE. The tank saying not to pull more would be no different than a DPS saying they don't want to use their AoE skills on the pack of a dozen mobs because they don't feel like it. Kick the tank, take the penalty, or let them kick you and wait for a healer, but don't think you're wrong for respecting the party's time. The OF is a very small percentage of the player base, so don't take its views as representative of the community as a whole.